Filters, in particular high-frequency filters, are used in a wide variety of electronic devices for many different applications and functions. For example, filters can be used to separate signals having different bands, such as a transmission signal and a reception signal, for example. One common implementation of a high-frequency filter that is used in wireless communications devices, for example, in the antenna duplexer of such devices, is a ladder-type filter such as that shown in FIG. 1.
FIG. 1 is a circuit diagram showing a configuration of an example of a ladder-type filter 100. The ladder-type filter 100 includes a plurality of series-arm resonators 104a-d connected in series with one another along a signal path between an input terminal 102 and an output terminal 103. The ladder-type filter 100 further includes a plurality of parallel-arm resonators 105a-c which connect respective nodes between the series-arm resonators 104a-d along the signal path to a ground via an inductor 106. In various examples, the series-arm resonators 104a-104d and the parallel-arm resonators 105a-105c of filter 100 may each be formed by one or more surface acoustic wave (SAW) elements. The SAW elements may include a resonator including an interdigital transducer (IDT) electrode and a pair of reflectors, all of which may be formed on a piezoelectric substrate made of lithium niobate or lithium tantalite, for example.
FIG. 2 is a diagram showing an electrode arrangement of a first example of a conventional SAW element 200. The top portion of FIG. 2 shows a plan view of the first conventional SAW element 200 including an interdigital transducer (IDT) electrode 210 and a pair of reflectors 220 disposed on either side of the IDT electrode. The IDT electrode 210 is formed by a pair of interdigitated comb-shaped electrodes. Each interdigitated comb-shaped electrode includes a busbar 212a, 212b, and a plurality of electrode fingers 214 extending therebetween. Adjacent electrode fingers 214 are alternately connected to the first busbar 212a or the second busbar 212b and interdigitate with one another. Each reflector 220 of the pair of reflectors includes a plurality of interdigitated reflector fingers 222.
The lower portion of FIG. 2 is a graph showing the electrode finger “pitch” (represented by P(x)) in the various regions of the first conventional SAW element 200. The horizontal axis of the graph represents the x-direction, which is the direction in which a surface acoustic wave propagates through the SAW element 200, and the vertical axis represents the pitch. As shown, in the first conventional SAW element 200, the electrode fingers 214 of the IDT electrode 210 are arranged with a first pitch P1, whereas the electrode fingers 222 of the reflectors 220 are arranged with a second pitch P2 greater than the first pitch P1.
FIG. 3 is an enlarged cross-sectional view of a part the first conventional SAW element 200, including a portion of the IDT electrode 210, taken along the x-direction and a thickness direction of a piezoelectric substrate 230 on which the IDT electrode 210 is formed. As shown, the electrode finger pitch P(x) can be defined as a distance from one side of an electrode finger 214b to a corresponding side of an adjacent electrode finger 214a of the IDT electrode. The same definition of pitch is applicable to the electrode fingers 222 of the reflectors 220 shown in FIG. 2. Thus, a greater pitch means that the electrode fingers 214 or 222 are spaced further apart from one another relative to a lesser pitch.
During operation, the IDT electrode 210 within the SAW element 200 is provided with an excitation voltage having one or more frequency components. The resonance characteristics of the SAW element 200 affect its response to the various frequency components of the excitation voltage. Specifically, the amount of impedance presented by the SAW element 200 varies with frequency in accordance with its resonance characteristics, and in turn determines the amount of signal attenuation at various excitation voltage frequencies.
The resonance characteristics of a SAW element are determined in part by its resonant frequency and antiresonant frequency. Excitation voltage frequencies close to the resonant frequency of the SAW element experience a relatively low amount of signal attenuation through the SAW element. A relatively narrow frequency band around the resonant frequency can be referred to as the “main lobe” of the frequency response (i.e., signal attenuation as a function of frequency) of the SAW element. In contrast, excitation voltage frequencies near the antiresonant frequency of the SAW element experience a relatively high amount of signal attenuation through the SAW element. The frequency response of the SAW element may additionally include one or more “side-lobes” corresponding to frequency bands in which excitation voltages experience an intermediate amount of signal attenuation through the SAW element, the amount being greater than that of the main-lobe, but still relatively low. Accordingly, excitation voltage frequencies within the main-lobe or side-lobes are significantly less attenuated by the SAW element than other excitation voltage frequencies.
Techniques for controlling the electromechanical coupling coefficient of a SAW element can be applied to alter its resonance characteristics, which affect its overall frequency response, as described above. For example, one technique for altering the electromechanical coupling coefficient of the SAW element 200 is to remove some of the electrode fingers 214 from the IDT electrode 210. The number and location(s) of the electrode fingers 214 that are removed can be selected so as to apply a “weighting” to suppress a first side-lobe of the frequency response of the SAW element 200.
FIG. 4 is a diagram showing an electrode arrangement of a second example of a conventional SAW element 300. The top portion of FIG. 4 shows a plan view of the second conventional SAW element 300 including an IDT electrode 310 and a pair of reflectors 320 disposed on either side of the IDT electrode. The second conventional SAW element 300 is similar to the first conventional SAW element 200 shown in FIG. 2, except for the addition of a technique of reversing the connections of some electrode fingers 314, as described below. Similar to FIG. 2, the lower portion of FIG. 4 is a graph showing the electrode finger pitch along the x-direction. As shown, in the second conventional SAW element 300, the electrode fingers 314 of the IDT electrode 310 are arranged with a first pitch P1, whereas the electrode fingers 322 of the reflectors 320 are arranged with a second pitch P2 greater than the first pitch P1.
The IDT electrode 310 of the second conventional SAW element 300 includes two regions 313 in which the electrode finger connections are reversed relative to the remaining region 311 of the IDT electrode 310. These “reversed regions” 313 are arranged symmetrically along the x-axis between the points m±d1 and each respective end of the IDT electrode 310. In the reversed regions 313, the electrode finger connections to the first busbar 312a and the second busbar 312b are reversed relative to the “normal region” 311. The reversed regions 313 begin at the points m±d1 along the x-axis and extend outward in each x-direction to the respective ends of the IDT electrode 310. In the reversed regions 313, the alternating pattern of electrode finger 314 connections established in the “normal region” 311 is inverted. Specifically, the first IDT electrode finger 314 in each reversed region 313 after crossing boundary m+d1 or m−d1 connects to the same busbar 312a, 312b as each previous IDT electrode finger 314 in the normal region 311 adjacent to the boundary. In other words, at each boundary m+d1, m−d1 there is a single repetition in the pattern of electrode finger connections that deviates from the alternating pattern of finger connections established in the normal region 311. The electrode fingers 314 having the reversed connections are hatched for clarity. The remaining elements are identical to those of the conventional SAW element 200 shown in FIG. 2.
FIG. 5 is a graph comparing resonance characteristics of the conventional SAW elements 200 and 300 shown in FIG. 2 and FIG. 4, respectively. The horizontal axis represents frequency, and the vertical axis represents the imaginary part of the impedance (reactive impedance) presented by each SAW element. In FIG. 5, the solid line A indicates the resonance characteristics of the conventional SAW element 200 of the first example (FIG. 2), whereas the dashed line B indicates the resonance characteristics of the conventional SAW element 300 of the second example (FIG. 4). The antiresonant frequency fB associated with the second SAW element 300 is shifted toward lower frequencies with respect to the antiresonant frequency fA associated with the first SAW element 200. Further, spurious emissions S can be observed in some regions of lower frequencies for the second conventional SAW element 300. These spurious emissions S result from non-periodicity caused by the reversed connections of the electrode fingers 314 implemented in the second conventional SAW element 300 of FIG. 4.
FIGS. 6A-6B are a set of graphs showing frequency characteristics of two examples of the ladder-type filter 100 implemented using the conventional SAW elements 200 and 300 shown in FIG. 2 and FIG. 4, respectively. In FIG. 6A, the solid line A indicates frequency characteristics of a first example of the filter 100 implemented using the conventional SAW element 200 of the first example (FIG. 2) as the series-arm resonators 104a-104d and as the parallel-arm resonators 105a-105c. The dashed line B indicates frequency characteristics of a second example of the filter 100 in which the series-arm resonators 104a-104d are each replaced with the conventional SAW element 300 of the second example (FIG. 4).
In FIG. 6A, a passband R1 and a stopband R2 of the examples of the filter 100 are shown. The second example of the filter 100, indicated by the dashed line B, shows an improvement of signal level in the stopband R2 compared to the first example of the filter 100 indicated by the solid line A. In particular, the stopband R2 of the second example of the filter 100 begins closer to the passband R1, or at a lower frequency, relative to the first example of the filter 100, as may be seen by comparing solid line A and dashed line B, and the corresponding signal level reaches an attenuation level of around −50 [dB] at a lower frequency relative to the first example of the filter.
FIG. 6B shows an enlarged view of a portion of the frequency range depicted in FIG. 6A, including the passband R1 of the second example of the filter 100. As can be seen in the passband R1, spurious emissions S occur in the second example of the filter 100, which uses the conventional SAW elements 300 of FIG. 4 (indicated by the dashed line B). As discussed above, these spurious emissions S result from non-periodicity caused by the inclusion of reversed IDT electrode finger connections in the reversed regions 313.